This can be done in any game mode, though Survival is the easiest. Start a Survival match on the Port, you can do this by yourself with just the AI. Run across to the bar from the generator and pick up the M60, return back to where you started and pick up a pipe bomb. Alert the horde and throw the pipe bomb in their direction. You need to be quick, so once throw the pipe bomb change over to the M60 and kill all the Infected and the achievement should pop.
If you don't manage to kill enough in one go, but still have ammo don't let go of the trigger. Keep firing at the rest of the Infected around you until the achievement unlocks. If you don't succeed call a vote and try again.